Primer coats for aircraft are required to withstand 3000 hours saltspray resistance with a deposition of about 15 microns (about 0.6 mil), and to avoid osmotic blister formation when topcoated and subjected to 168 hours immersion in water at 150.degree. F. (about 65.degree. C.). This is referred to as compatibility. Osmotic blisters are caused by a rapid loss of weight as the corrosion inhibiting agents dissolve, leaving numerous voids in the primer coating which result in a collapse of the topcoat. Saltspray resistance is determined by the test set forth in Northrop Corporation Material Specification NAI-1269 Revision G.
Since conventional aircraft primers contain corrosion inhibiting pigments such as chromates of zinc, strontium, cobalt and/or barium, zinc phosphate, and the like, it has been necessary for the prior art to use an epoxy polymer of relatively high molecular weight, which is solid at room temperature, in order to cause the metal chromates to dissolve or "leach" gradually during the 168 hours water immersion test at 150.degree. F., or the comparable test of condensing humidity for 1550 hours at 120.degree. F. (about 50.degree. C.). These medium to high molecular weight epoxy polymers used in the prior art require relatively high levels of VOC, well above the current permissible maximum of 340 grams per liter.
United States Pat. No. 3,954,693, issued May 4, 1976, to C.C. Fong, is representative of prior art coating compositions for aircraft utilizing an epoxy resin binder with a polyisocyanate hardener of specific type, pigments such as zinc, cobalt, and strontium chromates, a suspending agent which may comprise pyrogenic silica in amounts up to 4% by weight of the epoxy resin, a catalyst, an epoxy-containing silane as a coupling agent, and volatile organic solvents ranging from about 40% to about 85% by volume of the composition. In all specific examples, the total solvent content was at least 40%.
In order to attain the higher solids levels needed to meet the low VOC requirements, a relatively low molecular weight epoxy polymer is required which is liquid at room temperature. However, the lower molecular weight of liquid epoxy polymers permits rapid leaching of the corrosion inhibiting agents (chromates and the like), thereby reducing saltspray resistance and causing blistering when topcoated with polyurethane and immersed in water at 150.degree. F. for 168 hours.
This dilemma has made it impossible for conventional coating compositions simultaneously to meet the saltspray resistance and 150.degree. F. water immersion tests and to comply with the low VOC requirements. Consequently, the aircraft and aerospace industry has been operating under a waiver of the low VOC requirement, pending development of a primer composition which would effectively meet all requirements.
The above problems are further complicated by recent restrictions on the use of zinc and strontium chromates due to the carcinogenic nature thereof. Moreover, zinc phosphate has been found to be less effective as a corrosion inhibiting agent in conventional compositions.
It is known that the length of path which moisture must diffuse to penetrate a protective coating on a metal surface is greater for lamellar or flakelike pigment particles than a coating containing amorphous pigment particles. The platelike pigment structure provides a tight film barrier which inhibits the passage of moisture and atmospheric contaminants.
Naturally occurring metal oxides, such as micaceous iron oxide with particle sizes ranging from about 15 to 100 microns, have been applied with about 20 layers of interleaved lamellar particles (about 5 mils in thickness) to form a layered protective coating which is extremely moisture resistant. However, this prior art type of pigmentation is not suitable in protective coatings for aircraft, automotive components and electrical equipment, since such products must be coated in production lines with depositions between about 0.5 and 1.5 mils (12.5 and 40 microns) in thickness. In order to obtain a smooth topcoat of about 6 to 8 Hegman gauge, it is essential that the primer be milled to a fine grind. Consequently, the penetration of moisture must be inhibited by pigmentation other than a high build, rough lamellar particle interleave.
It is, therefore, evident that a genuine need exists for coatings of all conventional types which exhibit improved resistance against environmental attack and, in particular, primer coatings in the aircraft and aerospace field.